Heating it absolutely does have the chance to kill it, but I do believe it is your only option. It sounds like a board related issue, meaning one lr potentially multiple of the many components is failing. Heating it will allow the solder to reflow and potentially rebridge a connection that may have broke, but even that isn’t a permanent fix. I would do your best to recover any important data, that phone is well beyond the point of repair and anything that will get it working properly will not be stable for long term use. She needs a new phone.